Just hours before President Trump's "Salute to America" event was scheduled to kick off ... the rain started coming down HARD.

As we reported ... Trump said the show is expected to feature "large scale flyovers of the most modern and advanced aircraft anywhere in the world." POTUS also said it's possible Air Force One could make a "low and loud sprint over the crowd."

But, not long before the festivities got underway, rain crashed the shindig ... sending the crowd to look for cover. Even a small group of protesters hid under a nearby stage.

Tanks were on display near the Lincoln Memorial and are not gonna parade along Pennsylvania Avenue. The Navy's elite airshow squadron, Blue Angels, will do a flyover and will also be joined by other aircraft.

Trump -- who tweeted "people are coming from far and wide" -- will address the country from the Lincoln Memorial steps before a incredible 35-minute firework display slated for around 6 PM PT ... assuming the weather cooperates. The weather service says storms are expected this evening.

As we first reported .... the military-themed bash is costing taxpayers tons of dough. The portable toilet rentals alone cost a small fortune ... with the federal government shelling out 2 separate payments of $31,764.24 and $9,534. Golf cart expenses also racked up a $19,282 bill.

In case you were wondering ... Uncle Sam forked over $320,549.26 to Garden State Fireworks for the sparkly show. That's a jump from last year's cost of only $270k.